Solar Water Heating--Revised & Expanded Edition. Bob Ramlow

       Tanks with Built-In Heat Exchangers

      Some tanks are constructed with a heat exchanger as an integral part. Commonly referred to as indirect tanks, these have been popularized by the heating and plumbing industry as a means to heat domestic hot water with a separate hydronic heat source, typically a boiler. It used to be the case that tanks with internal heat exchangers were a specialized product to the solar thermal industry, making them very expensive. However, their application in the more traditional heating and plumbing field has expanded their usage and, consequently, driven the price down significantly.

      There are several indirect tank configurations for you to consider. The most common setup is to have a knurled or rifled copper or stainless steel coil of tubing placed inside a steel tank. This provides the greatest surface area in contact between the heat exchange coil and the domestic water, increasing efficiency. However, over time the textured surface of the coil may become coated with scale and mineral deposits, decreasing efficiency. This is especially true in hard-water situations. As long as the tank is flushed at regular intervals or installed in systems with soft or softened domestic water, it will be successful.

To avoid issues with scaling, some manufacturers of indirect tanks place the coiled heat exchanger on the outside of the tank. An illustration of the wraparound method can be seen in Figure 4.1. This technique reduces the amount of surface area that is in contact with the water you are heating but ensures consistent and prolonged efficiency of the tank. Instead of a coil, a few manufacturers use a fluid-filled jacket surrounding the tank. Essentially, the domestic water tank is submersed inside a slightly larger tank. In areas with hard water either one of these methods would be preferable.

      Some tanks have an internal heat exchanger that is removable and is accessible through a bulkhead fitting on the side of the tank. You can also buy a heat exchange “wand” that can be installed through the top port of standard electric or gas water heater. A removable heat exchanger allows for easy access and cleaning, but may limit the total amount of heat exchange surface area that can fit into a single tank.

      Indirect types of solar storage tanks have several advantages. First, installation is significantly simplified because you don’t have to do the extra work of mounting and plumbing an external heat exchanger. Though indirect tanks are more expensive, you must remember that you are eliminating the cost of an external heat exchanger and the cost to install it. Second, the system will require only a single main circulating pump. Most external heat exchangers require a pump on the solar side and on the domestic hot water side. The second pump results in not only an initial installation cost but also increased parasite loads and utility bills over time. Finally, the tanks can also save space where quarters are tight. After years of working in cramped utility rooms, and sometimes closets, we have developed an understanding of the value of floor space. Sometimes, this type of tank will be the only option.

Figure 4.1: Solar storage tank with wraparound heat exchanger

      Figure 4.2: Solar storage tank with internal heat exchanger

      Figure 4.3: Drainback tank with internal heat exchanger

       Tanks without Built-In Heat Exchangers

      Storage tanks without built-in heat exchangers are the most common kind found in solar water heating systems. These can be made of steel or fiberglass. To function as a solar hot-water-storage tank, the tank has to be insulated and have suitable fittings to allow proper connections of the water supply, the heat exchanger and the drain. A number of manufacturers build storage tanks specially designed for this type of application, similar to water heating tanks but with extra ports to make it easy to install the solar plumbing. It is also possible to use a traditional water heater tank. These tanks are readily available at competitive prices and come in both fiberglass and steel. If you go this route, it is best to use an electric water heater. A gas water heater tank will most likely have a flue down the center, which can lead to more standby heat loss, as well as providing more surface area to develop leaks. More information on how configure a traditional electric water heater into a storage tank will be covered in Chapter 8.

      Sometimes folks want to buy a used tank when installing a system. If you choose to do that, you should inspect the tank carefully for corrosion. In most cases the used tank was taken out of service either because it leaked or was old and ready to leak. We do not suggest using used storage tanks.

       Drainback Tanks

      Drainback systems need specialized reservoir tanks in addition to the storage tanks. They are like small solar storage tanks and usually hold about 5 to 20 gallons. Some drainback tanks have a heat exchanger built into them. Many have a sight glass built in to the side of the tank for monitoring the liquid level inside the tank. These specialized tanks are available from collector manufacturers and solar specialty shops. As with any other tank, you want to look for glass-lined steel or fiberglass tanks with adequate insulation.

       Solar Space Heating Storage Tanks

      Solar space heating systems are just overgrown solar water heaters. Being overgrown, they need an overgrown storage tank — think big. The size will vary depending on house size and climate. It is not uncommon for these storage tanks to be in the 300-gallon to 500-gallon range. To achieve the required storage size you can use either one large tank or several smaller tanks plumbed together. One large tank will always be preferable. These tanks are made of steel, stainless steel, fiberglass or, occasionally, concrete. There have been many attempts over the years to build storage tanks in place and line them with plastic or rubber. These do not work and will fail. The liners in these tanks are not designed to withstand temperatures commonly encountered in the system. They will often become brittle and eventually leak and crack.

      One big problem with large storage tanks is getting them into a building and through doorways. Some manufacturers have made tanks that come in pieces and are assembled in place. These are usually made of fiberglass and either bolt together or are cold welded with epoxy. We suggest selecting a cold-welded tank that is made of heat-resistant fiberglass capable of withstanding at least 180°F in continuous operation.

Heat Exchangers

      Liquid-to-liquid heat exchangers are used in a solar water heating system to transfer the heat from the solar fluid into the domestic water.

      Most heat exchangers are single walled. A single-walled heat exchanger has a single membrane between the two fluids. This membrane could be made of copper, stainless steel, or (in very specific circumstances) Pex. Double-walled exchangers afford an extra layer of protection to the potable water in case the heat exchanger ever develops a leak. Many have a gap between the layers, commonly referred to as “positive leak detection,” so that a leak in a wall will clearly be visible, and the heat exchanger can be repaired or replaced. Single-walled heat exchangers are typically more efficient because the heat needs to be conducted through less material. However, some municipalities may require a double-walled heat exchanger whenever domestic water is involved in the exchange. Rules requiring double-walled heat exchangers are often a carryover from the early years of our industry, when toxic liquids were used as heat transfer fluids. There is no need for a double-walled exchanger if a nontoxic solar fluid is used.